Rectifier circuit arrangement for television purposes



Jan. 24, 1939. K. SCHLESINGER 2,144,778

RECTIFIER CIRCUIT. ARRANGEMENT FOR TELEVISION PURPOSES Filed May 20, 1937 2 Sheets-Sheet '1 Jan. 24, 1939; K. SCHLE SlNGE R 2,144,778 I RECTIFIER CIRCUIT ARRANGEMENT FOR I'EHLJZTV'ISIQN PURPOSES Filed May 20, 1957 2. Sheets-Sheet 2 I hue/72W:

V al-m Patented Jan. 24, 1939 UNITED STATES RECTIFIER CIRCUIT ARRANGEMENT FOR TELEVISION PURPOSES Kurt Schlesinger, Berlin, Germany, assignor to Radioaktiengesellschaft D. S. Loewe, Berlin- Steglitz, Germany Application May 20, 1937, Serial No. 143,791 In Germany May 27, 1936 9 Claims.

The present invention relates to rectifying circuit arrangements used for television purposes and relates particularly to compensation of the carrier frequency modulated with television signal and simultaneously to the use of special circuit elements in the line between the rectifier and the succeeding amplifier and the image reproducing device respectively.

For rectification of the carrier frequencies in the case of television receivers there may be used both the grid leak rectifier as well as the anode bend rectifier. The grid-leak rectifier connected in special manner has a more favourable efficiency than the anode bend rectifier, particularly in the case of using an image having a number of lines in the order of magnitude of up to 200' lines. At a very high number of lines (400 lines) its effect decreases, as the grid capacities, which require to be charged, cannot be made as small as desired. In the case of lines in this number the anode bend rectifier possesses advantages in relation to the grid-leak rectifier (audion), as no condensers require to be charged.

The use of the anode bend rectifier for highly defined television images is known per se. Particular difiiculties do not occur in the normal circuit of a single anode bend rectifier having a single grid, so long as the carrier frequencies are very different from the modulating frequencies. As soon as, however, for the sake of a good intermediate-frequency amplification per stage, the carrier frequency is reduced to 1.5-2 times the image point frequency, receivers with grid leak rectification tend very greatly to self-oscillating and to reaction effects respectively. This is due to the fact that the lower side-band frequencies of the carrier occur in the anode circuit of the anode-bend rectifier and already extend into the frequency spectrum of the image frequencies. Since the anode-bend rectifier itself already has an amplifying effect, and usually an output amplifier is additionally used for the image frequencies, the highest image-frequencies occur in the anode circuit thereof in 10-50-fold amplification and can effect in combination with the low side-band frequencies of the carrier a reaction coupling, even if this amplifier is stabilized itself.

The subject matter of the present invention relates to an embodiment of an anode-bend rectifier circuit connected in push-pull more particularly for television arrangement operating with low intermediate frequencies having simultaneously .in its anode circuit, which is coupled to the succeeding amplifier stage and the image reproducing device respectively oscillatory choke combinations, for example, a filter chain circuit which suppresses self-oscillating or reaction effects in the frequency range near the highest image modulation-frequencies and lower sideband frequencies of the carrier. A further object of the invention is to prevent detrimental effects in the amplifier circuit succeeding to the rectifier circuit caused by fluctuations in the grid bias potentials and in the anode circuit potential produced by strong variations of the rectified impulses in the anode circuit of the rectifier.

Further objects of the present invention will be clearlyunderstood by way of circuit embodiment shown in the accompanying drawings in which b Fig. 1 shows a rectifier circuit with its associated circuit arrangement in accordance with the invention.

Fig. 2 shows an embodiment of the input circuit to the rectifier circuit showing a special coupling transformer arrangement.

Fig. 3 shows a frequency diagram in which the frequency range of the image modulation frequency-band and the side-band of the carrier is illustrated particularly, the range is to be recognized in which detrimental reaction effects are arising.

' Fig. 4 shows a further embodiment of the rectifier circuit and its associated circuits in accordance with the invention.

The circuits according to the invention are only of special importance in television arrangements operating with low intermediate frequencies that means with intermediate frequencies which amount to merely approximately twice the image frequency. A low intermediate frequency of this kind is advisable on account of the optimum amplification thus obtainable. The side band of the carrier frequency fT amounts to 2'7max extending from 3v to 1- (see Fig. 3). On the other hand the amplifier behind the rectifier should have full amplification from 'Ymax to zero. Therefore without using the measures according to the invention areaction coupling takes place in the vicinity of 'y.

In Fig. 1 the final intermediate-frequency amplifying tube I operates by way of the transformer 2 a special rectifying tube 3, which contains two separate grids with common cathode and anode. The two grid paths operate independently of each other as anode bend rectifier in push-pull. For this purpose they require to be arranged side by side over a common cathode, i. e., should not be wound one within the other,

as otherwise there would be prevented an anodebend rectifying effect. In principle, therefore, the tube 3 may also be replaced by two completely separate three-pole tubes having parallel-connected cathods and anodes.

In the unexcited condition the anode current is interrupted by the bias 4. Upon excitation of the rectifier the rectifying effects are added together, whilst the fundamental wave in the anode circuit is eliminated and there occur in the anode circuit only frequencies commencing with the octave of the exciting frequency. In this way there is already avoided an interference of the maximum image frequency with the lowest side band frequency of the carrier. Nevertheless reaction coupling occasionally occurs if it is not ensured in accordance with the invention that the amplification now following has a strong drop in amplification at its maximum frequency. This is obtained by the oscillatory choke connections in accordance with the invention, having the chokes 5 and 5 and the damping resistances I, 8 or 9, l0 and employed in the grid and anode circuits of the associated amplifier I I. At the same time the amplifier I l, whilst avoiding condensers, is connected galvanically to the detector anode circuit, and its anode circuit is also in galvanic connection both with the line to the image receiver B as well as with the line to the synchronizing element S. The correct grid bias is regulated as total potential M; at this regulation the tapping ratio 4:14 must remain constant. In mains-connection systems, therefore, the potential M will be represented by a fixed resistance ratio, from which there is tapped the part-potential 4 and which is excited by variable current for the purpose of adjusting the correct bias in respect of II.

The oscillatory chokes 5 and 6 are so dimensioned that in the vicinity of the highest image frequency they produce a resonance effect with the associated gridor line capacities. The damping resistances I and 8 are so determined that the damping lying in parallel to 5 represented by their total is sufficient to keep the resonance effect within desired limits, whilst the working resistance for lower frequencies is provided by the parallel connection of 1 and 8.

There will be found according to the inven tion, as most convenient dimensioning, 7:8 and 9:10, both being in themselves twice as great as the desired working resistance in the case of low image frequencies. Several choke and resistance combinations of this kind with resonance at 'Ymax or somewhat higher than 'ymax may also be connected in series. They improve the flank drop of the transmission curve of the amplifier and decrease the tendency of the entire receiving system towards reaction coupling.

It has been found in practice that it was only with the assistance of both measures, i. e., pushpull operation on the one hand and oscillatory choke combination on the other hand, that it was possible to avoid in reliable fashion self-excitation. Operations with a single-anode-bend rectifier were only possible with a ratio: carrier frequency: video frequency 25. For adjusting the correct push-pull operation it is desirable to arrange the secondary coil of 2 to be shiftable and to select a special transformer construction, which is illustrated in Fig. 2. The secondary coil l2 of the coupling transformer is closely wound, whilst the primary coils l3a, l3b are connected in series and arranged on either side of the primary coil l2. By displacement of the coil l2 in an axial direction the capacitative coupling between I2 and Ba, I3!) is varied, and since the same increases the coupling effect in respect of 13a and decreases the coupling effect in respect of I31) (or vice versa, dependent on the direction of winding of l2), there may accordingly be attained a very accurate adjustment of the phase potentials.

Fig. 3 explains the frequency conditions in the case of the use of low intermediate frequency having a value of nearly twice the highest image frequency. The band of the end amplifier extends from the frequency 0 up to the mark 15, i. e., 'Ymax. The carrier frequency is located at [6 in the amount fT=2"ymax. The side band of this carrier then extends from IE to I! and is represented by the rectangle a, b, c, d. Since amplifiers do not exist which have an infinitely steep drop of the transmission curve at a definite frequency, the amplifier must drop according to I the curve 6. It accordingly overlaps in the shaded zone with the lower side band frequencies a/b of the carrier. At this point self-excitation is unavoidable. Remedy is obtained either by shifting the entire carrier frequency spectrum into a range of much shorter waves, i. e., to a higher frequency range resulting in amplification losses, or, in accordance with the invention, by compensation of the carrier frequency on the one hand and sharpening the drop in frequency by an absorption circuit with oscillatory chokes, which improve the flank steepness of the curve e into such as shown by curve I.

Fig. 4 shows a further embodiment of the invention which consists in using special circuit arrangements additionally in the amplifier circuit associated to the rectifier and in the rectifier circuit itself.

It is to be observed in the case of galvanic connecting rectifier and succeeding amplifier, which is obsolutely essential for modern television reception purposes, that the maximum anode current value of the end stage is shifted upon the arrival of powerful signals. This is not permissible, as in conjunction with galvanically coupled synchronization it leads to a tearing of the image in the case of powerful reception. According to the invention, means are described for compensating this interference. There is also a need for the possibility of regulating the maximum oscillation amplitudes supplied by a rectifier of this character, as it is desired to employ detectors of this kind for a variety of purposes, for example for cable circuits in combination with television tubes sensitive and insensitive, projection tubes and the like. If an adjustability of this kind does not exist, there is the danger of over-control and under-control.

In Fig. 4 there is shown the circuit diagram of the improved rectifier arrangement. For reasons associated with a constant condition of the screening grid potential the end pentode H must be directly connected by means of its screening grid with the anode potential lead l8. The same, therefore, requires a bias, which is adjusted at the resistance l9. If now a powerful anode current takes effect from the rectifier 3, the anode current of the end stage II sinks and the image changes over towards white. By reason of the lost emissive current, however, the bias of the control grid of H increases in its mean value. If, therefore, the detector 3 is de-energized in high-frequency respect for a brief period, for example upon the synchronization impulses, the anode current of l I now increases beyond the value which existed in the stationary condition before the apparatus was set into operation. If,

I therefore, a synchronization diode 25 having a fixed bias was connected with the anode II, the synchronization impulse will now be reproduced too strongly and the image will be interfered with. According to the invention, there is provided in series with the high-frequency working resistance of the detector 3, which consists of a resistance combination I, 8 and a choke with transmission limit in the vicinity'of the maximum image point frequencies, a correcting resistance 22 with parallel condenser 21. The correcting resistance 22 may be readily adjusted in such fashion that the loss of bias at the resistance I9 is compensated by a gain of negative bias 22.

This condition applied in respect of direct currents, but only to medium-frequency currents if the time constant for 21/22 is equal to the time constant for 19/20. The result is that the hight of the negative impulse peaks in the anode current diagram of l remains completely independent of the excitation of the detector 3 and constant.

The desire for adjustability of the maximum oscillation potentials supplied by a rectifier of this nature is complied with as follows according to the invention by means of a bridge connection: As well known, a rectifying amplifier 3 operates in the point of initiation of the anode current. This is adjusted by a potentiometer bridge 23. By displacement of the tapping at 23 towards the right the anode current may just be suppressed and upon further displacement remains at zero. The adjustment of the tapping of 23 corresponds to the reciprocal ratio of the tube 3. If this value has once been adjusted and if the tube is Without current, the operating potential 25, in accordance with the invention, may now be varied at a potentiometer 24 without variation of the conditions relating to the initiation of the anode current of 3. Since the current through 1, 8, 22 remains unchanged at zero the current through the end stage is also not varied, as the grid bias of the end stage continues to exist. It is possible, therefore, to adjust independently at 24 the maximum anode potential of the detector 3, the working characteristic of which also remains unchanged. There is accordingly then also adjustable the maximum grid alternating potential capable of being supplied to II. It is at the most equal to the direct potential adjusted at 24. Any excess beyond this value and accordingly an over-control of the image receiver connected to line I 8 is impossible. In order to be able to utilize completely the bias 24 it may be preferable to employ as detector 3 a pentode in which the anode is capable of oscillation down to the potential of the cathode. There have been included the screening grid 21 and the intercepting grid 28, the screening grid being applied to the full anode potential, 1. e., in this case to earth.

The detrimental variation of the medium grid bias at H! becomes smaller if I9 is traversed not only by the end stage current but also by other currents which do not vary. Conveniently all other tube currents of the receiver can be passed through l9. I9 is then correspondingly smaller. The correcting resistance 22 then also immediately becomes smaller, and the correction of a correspondingly smaller interference is accordingly more simple.

I claim:

1. A television receiver including a final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency,

said rectifier comprising a, push pull anode bend detector with a filter circuit in its output circuit, said filter circuit having a high impedance at the maximum image frequency.

2. A television receiver including a final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency, said rectifier stage comprising a rectifier tube operating as anode-bend rectifier with two separate grids between a common cathode and the anode, a filter circuit in the anode circuit of said rectifier having a high impedance at the maximum image frequency, each of said two grids controlling independently of the other, that part of the cathode which is adjacent thereto, said grids and said anode with its lead in connections being of low capacity.

3. A television receiver including a final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency, said rectifier comprising a push-pull anode bend detector with a filter circuit in its output circuit, said filter circuit having a high impedance at the maximum image frequency and consisting of a coil with an associated series damping resistance in combination with a parallel resistance which is tapped at its centre and has a value equal to twice the operating resistance required for the lower image frequencies.

4; A television receiver including a final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency, said rectifier comprising a push-pull anode bend detector with a filter circuit in its output circuit, said filter circuit having a high impedance at the maximum image frequency and consisting of a coil with inherent damping resistance in combination with a parallel resistance which is tapped at its centre and has a value equal to twice the operating resistance required for the lower image frequencies.

5. A television receiver including a final rectifier stage, an associated amplifier stage and an image reproducing device coupled thereto, said final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency comprising a push-pull anode bend rectifier, the connection between said rectifier, said amplifier and said image reproducing device being performed galvanically and consisting each of a filter circuit which has a high impedance at the maximum image frequency and consists of a damped coil in combination with a parallel re sistance which is tapped at its centre, and has a value equal to twice the operating resistance required for the lower image frequencies.

6. A television receiver including a final rectifier stage, an associated amplifier stage, an image reproducing device coupled thereto, a filter circuit between said single stages, a correcting resistance and a biassing resistance, said final rectifier stage being tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency and comprising a push-pull anode bend rectifier, said filter circuit connected between said rectifier and amplifier stage having a high impedance at the maximum image frequency and consisting of a damped coil in combination with a parallel resistance which is tapped at its centre and having a value equal to twice the operating resistance required for the lower image frequencies, said correcting resistance being connected in series with said filter circuit and earth and having a condenser connected in parallel to its terminals which is so dimensioned as to compensate for changesin grid, bias of said amplifier stage arising due to changes in the anode current of said amplifier stage, said 'biassing resistance connected between cathode of said amplifier and earth being also by-passed by a condenser, which has such magnitude that the time constants of said two resistance-capacity combinations are equal to one another.

7. A television receiver including a final rectifier stage with a coupling transformer in its input circuit, an associated amplifier stage and an image reproducing device coupled thereto, said final rectifier stage tuned to carrier frequency of about 1.5 to 2.0 times the maximum image frequency comprising a push-pull anode bend rectifier, the connection between said rectifier, said amplifier and said image reproducing device being performed galvanically and consisting each of a filter circuit which has a high impedance at the maximum image frequency and consists of a damped coil in combination with a parallel resistance which is tapped at its centre, and has a value equal to twice the operating resistance required for the lower image frequencies, the primary and secondary of said input transformer With the associated inherent capacities being tuned to thesame or approximately the same frequency as the carrier, said primary being divided into two parts arranged on either side of the secondary which is axially shiftable between said primary parts.

8. A television receiver including a final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency, said rectifier stage comprising a rectifier tube operating as anode-bend rectifier with two separate grids between a common cathode and the anode, a filter circuit in the anode circuit of said rectifier having a high impedance at the maximum image frequency, each of said two grids controlling independently of the other that part of the cathode which is adjacent thereto, said grids and said anode with its lead in connections being of low capacity, the cathode of said rectifier stage having a specially insulated cathode coating which is arranged on the same common heating element and serves in combination with a special plate electrode as synchronizing system.

9. A television receiver including a final rectifier stage with a coupling transformer in its input circuit, a potentiometer resistance in the anode circuit, an associated amplifier stage and an image reproducing device coupled thereto, said final rectifier stage tuned to a carrier frequency of about 1.5 to 2.0 times the maximum image frequency comprising a push-pull anode bend rectifier, the connection between said rectifier, said amplifier and said image reproducing device being performed galvanically and consisting each of a filter circuit which has a high impedance at the maximum image frequency and consists of a damped coil in combination with a parallel resistance which is tapped at its centre, and has a value equal to twice the operating resistance required for the lower image frequencies, the primary and secondary of said input transformer with the associated inherent capacities being tuned to the same or approximately the same frequency as the carrier, said primary being divided into two parts arranged on either side of the secondary which is axially shiftable between said primary parts, said potentiometer resistance being adjusted to suppress the anode current of the detector the grid bias of said rectifier stage being also tapped from said variable potentiometer.

KURT SCHLESINGER. 

